In the field of electronic components, decorating items or the like, resin materials with a metal film pattern formed thereon have been used from many years ago. Typical examples thereof include a flexible printed-wiring board wherein a metal film circuit pattern is formed on the surface of a resin film.
Along with the miniaturization of electronic equipment, the miniaturization of flexible printed-wiring boards has also been developed and it has been required to form a finer circuit pattern. At the same time, in addition, it has also been required to improve the adhesiveness of the metal film to the resin film surface.
As a typical method for forming a circuit pattern, it has been known a method wherein a metal film is formed on a whole surface of a polyimide resin film by means of vapor deposition, sputtering or laminating a copper foil using an adhesive, then the metal film is exposed in pattern by a photolithography method, being developed, and the unnecessary metal is removed by etching.
However, this method has such problems as a low productivity cause by the great mass of metal to be removed, heavy effects on the environment caused by etching waste liquid and high cost of equipment and material such as photomasks, exposure devices, and photoresist.
Attention has been focused on a pattern printing technology using a metal nano-ink. The metal nano-ink and/or paste can be patterned on a substrate by an inkjet printing method or a screen printing method, whereby a conductive pattern can be formed directly thereon. However, the method also has such problems that the cost of raw material is high and that a calcination process is required after printing and therefore the substrate to be used is restricted to be one of heat resistant. In addition, specific resistance of the conductive pattern thus formed is higher compared with that formed by plating, which may cause a problem from the aspect of electric characteristics for applications for electronic equipment.
As a means for solving the above problems, in recent years, attention has been focused on a direct metallization method as disclosed in Patent Documents 1, 2 and 3, in which an imide ring of a polyimide resin film is opened by treating the surface of the polyimide resin film with alkali, then metal salts are adsorbed to the carboxyl group formed by opening the imide ring, and the metal salts are reduced to deposit a metal film.
Patent Documents 4 and 5 disclose a method wherein an alkaline ink is applied by an inkjet method selectively to a polyimide resin substrate at the part for forming the inorganic thin film to open the imide ring, then metal ions are adsorbed to said part to form metal salts, and then the metal salts are reduced to form an inorganic thin film pattern.
According to the method, it is not necessary to remove the metal film of the unnecessary part by etching after forming a metal film on the whole surface of the polyimide resin substrate.